Prepare for cruise with letter on outside of trunk (6)
I believe the answer is:
embark
'prepare for cruise' is the definition.
I can't judge whether this defines the answer.
'with letter on outside of trunk' is the wordplay.
'with' says to put letters next to each other.
'letter' becomes 'em' (I've seen this in other clues).
'on outside of trunk' becomes 'bark' (I can't explain this - if you can you should give a lot more credence to this answer).
'em' put next to 'bark' is 'EMBARK'.
Can you help me to learn more?
(Other definitions for embark that I've seen before include "Go aboard" , "Go on board ship or plane" , "Go on board, boat or plane" , "get on board" , "Start" .)